A rolling-bearing unit is used for supporting an automobile wheel so that it rotates freely with respect to the suspension. Moreover, in order to be able to control an anti-lock-brake system (ABS) or traction-control system (TCS), it is necessary to detect the rotation speed of the aforementioned wheel. In order to do this, a rolling-bearing unit with rotation speed sensor, in which a rotation speed detection device has been built into the rolling-bearing unit described above, has been widely used in recent years to support the wheel such that it rotates freely with respect to suspension, as well as to detect the rotation speed of the wheel.
FIGS. 1 and 2 show one example of a prior art rotation speed sensor, as disclosed in Japanese Patent Publication No. Toku Kai Hei 8-296634, that is used for the purpose described above. This rolling-bearing unit with rotation speed sensor comprises a stationary race or outer race 1 which does not rotate during use, a rotatable ring or hub 2 which is supported on the inside of this outer race 1 and which rotate during use, an encoder 3 which is fixed at one location on this hub 2, and a sensor 4 which is supported by the outer race 1 to detect the rotation speed of the encoder 3. In other words, stationary raceways or outer ring raceways 5 in double rows are formed around the inner peripheral surface, or stationary peripheral surface, of this outer race 1. Moreover, the hub 2 has a pair of inner races 7 which are fixedly fitted around the outer peripheral surface of the main hub body 6.
Rotating raceways or inner ring raceways 8a, 8b are formed around the rotatable peripheral surfaces or outer peripheral surfaces of both of these inner races 7, respectively. Multiple rolling members 9 are rotatably supported by a cage 10 in each row between the inner ring raceways 8a, 8b and outer ring raceways 5, so that they support the hub 2 so as to rotate freely inside the outer race 1.
Moreover, on the axially outer end of the main hub body 6 (end of the outside in the direction of width when installed in the automobile, left end in FIG. 1) in the area that sticks out in the axial direction from the axially outer end of the outer race 1, there is a flange 11 for attaching to the wheel. Also, on the axially inner end of the outer race 1 (end on the center side in the direction of width when installed in the automobile, right end in FIG. 1) there is an installation section 12 for attaching the outer race 1 to the suspension. Furthermore, the space between the opening on the axially outer end of the outer race 1 and the outer peripheral surface around the mid portion of the hub 2 is covered by a seal ring 13.
In order to install a rotation speed sensor in this kind of rolling-bearing unit, the encoder 3 is fitted around a portion closer to the axially inner end of the main hub body 6 that protrudes axially inward from both of the inner races 7. This encoder 3 is formed into a circular ring shape from a magnetic metal plate such as steel and provided with a detected portion 14 on the axially inner face (the right face in FIG. 1) at a portion closer to the outer periphery thereof. This encoder 3 is fitted around a portion closer to the axially inner end of the main hub body 6 and is held in place between a nut 15 that screws to the axially inner end of the main hub body 6, and the surface on the axially inner end face of the inside inner race 7.
The detected section 14 is formed with recesses and lands around in the circumferential direction, and is shaped with a gear-like shape, and the magnetic characteristics of this detected section 14 change alternately at evenly spaced intervals around in the circumferential direction.
Furthermore, a cylindrical-shaped cover 16 with a bottom fits into the opening on the axially inner end of the outer race 1 to cover the opening on the axially inner end of the outer race 1. This cover 16 is made from plastically worked sheet metal, and comprised of a cylindrical fitting section 17 that freely fits inside the opening on the axially inner end of the outer race 1, and a cover plate section 18 that covers the opening on this axially inner end of the cylindrical fitting section 17. A sensor 4 is supported at a radially outer portion in this cover plate section 18, and the tip end face (left end face in FIG. 1) of the detection section 19 of this sensor 4 faces the axially inner face of the detected section 14 of the encoder 3 in the axial direction through a small clearance, for example 0.5 mm.
In the case of the rolling-bearing unit with rotation speed sensor described above, the wheel that is attached to the flange 11 formed on the axially outer end of the hub 2 is able to rotate freely with respect to the suspension that supports the outer race 1. Moreover, as the encoder 3 that is fitted around the axially inner end of the hub 2 rotates with the rotation of the wheel, the lands and recesses formed on the detected section 14 alternately passes by the end surface of the detection section 19 of the sensor 4. As a result, the density of the magnetic flux that flows through the sensor 4 changes, and thus the output of the sensor 4 changes.
The frequency of the changing output of the sensor 4 is proportional to rotation speed of the wheel. Therefore, if the output from the sensor 4 is sent to a controller (not shown in the figure), it is possible to adequately control the ABS or TCS.
In order to secure the reliability in detecting the rotation speed of the wheel by a rolling-bearing unit with rotation speed sensor that functions as described above, it is necessary that the dimension of the gap between the tip end face of the detection section 19 of the sensor 4 and the end surface of the detected section 14 of the encoder 3 be stable. On the other hand, the components that make up the rolling-bearing unit deform elastically as the automobile operates. Particularly, as the automobile is turned fast, the amount of elastic deformation of the components increases due to the moment load that is applied to the hub 2 (by the turning acceleration) from the wheel by way of the flange 11. Also, due to the increase of the amount of this elastic deformation, the dimension of small clearance changes. This kind of change in dimension causes the output of the sensor 4 to change, and thus becomes a possible cause of loss of reliability of the rotation speed detection.
Therefore, in the case of the invention disclosed in Patent Publication No. Toku Kai He 8-296634, the sensor 4 is placed on a horizontal plane that passes through the center axis of the hub 2, so that with the changes in dimensions of the small clearance kept to a minimum, the reliability of the detected rotation speed is maintained regardless of elastic deformation of the components.
In the case of the invention disclosed in Japanese Patent Publication No. Toku Kai Hei 8-296634, only part of the elastic deformation of the components due to moment load applied to the hub 2 when the automobile turns fast is taken under consideration. Therefore, it is actually impossible to stabilize the dimension of the small clearance between the tip end face of the detection section 19 of the sensor 4 and the end surface of the detected section 14 of the encoder 3. In other words, due to the moment load, the displacement occurs in the rolling-bearing unit such that the center axis of the outer race 1 does not coincide with the center axis of the hub 2, and in addition, displacement that the outer race 1 and the hub 2 shift with reference to each other in the axial direction also occurs. The invention disclosed in Japanese Patent Publication No. Toku Kai Hei 8-296634 only takes into consideration the displacement that the center axis of the outer race 1 does not coincide with the center axis of the hub 2. Therefore, even though the sensor 4 is actually located on the horizontal plane that passes through the center axis of the hub 2, it is not possible to stabilize the dimension of the small clearance, and thus does not contribute much to maintaining the reliability of the detected rotation speed.